Potential of reactivity controlled compression ignition (RCCI) combustion coupled with variable valve timing (VVT) strategy for meeting Euro 6 emission regulations and high fuel efficiency in a heavy-duty diesel engine

As an effective strategy to control the combustion of advanced combustion modes, the application of variable valve timing (VVT) in reactivity controlled compression ignition (RCCI) combustion was investigated in this study. By coupling KIVA-3V code with genetic algorithm, the combustion of a heavy-duty engine with RCCI combustion combined with VVT strategy was optimized under a wide load range. At each load, six operating parameters including premix ratio (PR), intake valve closing (IVC) timing, start of injection, exhaust gas re-circulation rate, intake pressure, and intake temperature were optimized to realize low-emission and high-efficiency combustion. The optimization results indicate that, at low load, high PR coupled with either late IVC or base IVC can be utilized for the realization of high thermal efficiency. At mid load, the base IVC strategy is integrated with high PR, while the late IVC strategy is coupled with low PR. At high load, only the strategy with late IVC and low PR can be used. The strategy with higher PR and earlier IVC timing exhibits better engine performance on thermal efficiency and soot emissions, while the strategy with lower premix ratio and later IVC timing is superior in ringing intensity. By optimizing the RCCI combustion with the VVT strategy, the Euro 6 NOx limit can be met while maintaining ultra-low soot emissions at low and mid load. However, at least one after-treatment device is required to further eliminate the NOx or soot emissions at high load. Under the whole load conditions, satisfactory fuel consumption can be obtained.